Multicore cables in parallel - hysterisis (eddy currents)

Problems can occur with multicore cables, but only if there's an imbalance in the "going" and "return" currents in each cable. Normally such arrangements aren't permitted (521.8.1) and where cables are used in parallel things should be arranged so that there's a more-of-less equal division of currents - so again there should be a pretty good balance within each cable. I can see there may still be some small imbalance in practice - but as long as the imbalance is relatively small (<100A say) and we're talking thin steel sheet enclosures rather than 1/2" cast iron, the result is likely negligible (there is a suggestion that the whole 521.5.1 regulation can be safely ignored for DB where the imblance per hole is <100A (from memory).

   - Andy.

Hi Chris,

You're understanding is correct. While the hysteresis would be negligible the wording of 521.5.1 suggests all circuit conductors are collectively surrounded by ferromagnetic material. so my concern is with compliance of the wording, it may well be that 521.5.1 is written in a way that causes confusion and ought to be re-worded with a caveat for multicore cables in parallel?

I see what you mean. That would be a very literal interpretation. I have no problem with that, but in practice, there will be no difficulty.

Being too literal could cause problems with any conductors in SWA in parallel. Would you have to combine their wire armour using some form of twin gland at the point of entry?

Even the armour itself would count as a ferrous enclosure - applying that interpretation of 521.5.1 would mean that using SWA for parallel applications at all would be out of the question.

  - Andy.

+ ½

I had thought about that and I think that the armour does satisfy the definition of "enclosure", etc. So, if you combine two or more enclosures, they become one enclosure. (We can ignore the gap which a brass gland provides.)

Not really - the wires in the SW armour are neither  magentically in good contact nor forming a closed magnetic path enclosing a net current.. Had you said welded conduit, I'd have no problem with the statement.

I'm still not seeing the problem, so long as the cores in each cable form a complete circuit, that is to say that all 4 current carrying cores that carry associated current, are together.

Actually, for sensiible thicknesses of steel box, and sensible hole sizes you need many tens of amps going out of one hole and coming back via another to raise things by a few degrees.

Far more common, with unbalanced currents and it does apply to SWA or AWA as well, is the transformer-like problem of armours earthed at both ends forming a one turn loop for voltage pick up. Dog bone slots don't help much there, as that really is an eddy current effect .

Its why single core SWA does not exist....

Strictly, the problem we are discussing is not eddy current, rather it is magnetic hysteresis, though i appreciate even some publications that really should know better get confuse the two.

Mike.

Hi,

I'm not disputing the physics here. The question is compliance with BS7671.

Gareth

The question is compliance with BS7671.

The issue then is the word "collectively" - does it refer to all conductors of the circuit or merely some suitable collection of them. Certainly ambiguous (online dictionaries suggest both "as a group; as a whole" as a definition - so could be either way). Given the physics, "a suitable group" might not be an unreasonable interpretation.

Also consider how the reg could apply to other situations - say a steel adaptable box used as a JB for lighting - it would be quite normal to take just the L and SL (without N) through one hole for a switch drop, SL and N through another to the light - so neither contain "all" the conductors of the circuit - but do contain a suitable (balancing) group.

   - Andy.

AJJewsbury said:

Also consider how the reg could apply to other situations - say a steel adaptable box used as a JB for lighting - it would be quite normal to take just the L and SL (without N) through one hole for a switch drop, SL and N through another to the light - so neither contain "all" the conductors of the circuit - but do contain a suitable (balancing) group.

But 521.5.1 does not say that all of the conductors must enter (or leave) together. It simply says that the ones which do must go through the same hole.

Hi All,

Thanks all for your comments!

I have just spoken to the technical helpline at the NICEIC who have confirmed that all conductors of the circuit (even consisting of multicore cables) must enter the same aperture of a ferrous enclosure in order to comply with BS7671.

Cheers

Gareth

Again  this falls back to definitions, in this case 'circuit' which seems to mean something different in standards land, to the meaning for the broader world of electron herding. Current flowing out to a light switch and back does form a circuit in an electrical sense. - but in standards speak is only part of a circuit.

Again  this falls back to definitions, in this case 'circuit' which seems to mean something different in standards land, to the meaning for the broader world of electron herding. Current flowing out to a light switch and back does form a circuit in an electrical sense. - but in standards speak is only part of a circuit.